Investigation of the pathogenicity and persistence of Beauveria bassiana for biological control of Fuller's rose weevil larvae in soil

Abstract

The primary aim of this project was to investigate the potential for biological control of Naupactus cervinus, Fuller's rose weevil (FRW), in kiwifruit orchards, using entomopathogenic fungi. FRW was a pest on kiwifruit due to adult weevils laying eggs on the fruit. This was a quarantine issue for overseas markets, resulting in economic losses to the New Zealand kiwifruit industry. FRW larvae develop in the soil over several months, so investigations were focussed on the larval stage. Beauveria bassiana was the pathogen of primary interest as it is ubiquitous in soil and is known to infect immature coleoptera. Some study of Beauveria brongniartii and Metarhizium anisopliae was also made. In total, ten strains of B. bassiana, one strain of B. brongniartii and two strains of M. anisopliae from New Zealand, Chile, Romania and USA were investigated for pathogenicity towards FRW larvae.

Strain characterisation was performed in order to have a meaningful framework, in terms of phenotypic and genotypic differences, in which to place the selected strains. Strains were characterised on the basis of optimal temperatures for germination and conidiation, conidial size, and molecular analysis of the ITS region. The optimal temperature for germination and conidiation of most of the strains tested was between 20° and 25°C. Strains that had a high rate of germination and conidiation at 15°C were identified. No correlation between the size of conidia and virulence towards FRW larvae was found. The investigation of conidial swelling under different conditions indicated a positive relationship between the swelling of conidia in H₂O over time and environmental persistence. No relationship was suggested from the analysis of the ITS region between genotype and geographic or host origin.

Beauveria spp. and M. anisopliae strains can vary significantly in virulence towards a pest species. All thirteen strains were tested for pathogenicity towards FRW larvae in soil. All strains were pathogenic but varied in virulence. Strain F164 was consistently the most virulent towards FRW larvae. Fourth and fifth instar larvae were more susceptible to fungal infection than third instars. Larvae were less susceptible to fungal infection in native soil than in oven-dried soil. Conidial germination on FRW larvae cuticle was assessed and a positive relationship between germination rate and virulence was found.

Environmental persistence is a contributing factor to the success of biological control using entomopathogenic fungi. The persistence of conidia of strains of Beauveria spp. and M. anisopliae in soil was investigated in native orchard soil and oven-dried soil at 15° and 20°C. The B. bassiana strain, 8928, was consistently more persistent in soil than other strains. An assessment of conidial germination and viability in soil was made using fluorescent microscopy. Strains that had a low rate of germination in soil showed good long-term persistence.

This study has shown good potential for control of FRW larvae using B. bassiana. Furthermore, this work has broader application to investigations of biocontrol of soil-dwelling weevil pests.